1. Field of the Invention
The present invention relates to a silicon wafer and a production method therefor. More specifically, the present invention relates to a polished (mirror processed) silicon single crystal wafer (mirror wafer) which is doped with nitrogen, hydrogen and carbon, and comprises a void region, and method for producing the silicon wafer.
2. Background Art
Characteristics of the growth of a silicon single crystal by the Czochralski (“CZ”) method include (1) crystal growth from a free surface of a melt which is gravitationally stable, (2) doping or adjustment of impurities, which is necessary to adjust resistivity, taking into account a concentration gradient in an axial direction due to a segregation coefficient specific to the kind of impurity, (3) control of the oxygen concentration effective to getter metal impurities by dissolving oxygen from the crucible and control of the oxygen concentration or dopant concentration by control of the pulling conditions, (4) the use of a quartz crucible or graphite parts having higher purity and larger scale, and (5) use of a large sized apparatus which enables the growth of a large diameter silicon single crystal which is free of dislocations. As large size silicon single crystals grown by the Czochralski method are used as a substrate for LSI, severe control of the quality of the silicon crystal is required.
Grown-in defects existing inside a crystal just after silicon single crystal growth, oxygen precipitates, dislocations induced in device production process, and stacking faults, etc. deteriorate device characteristics. On the other hand, it is known that oxygen precipitates can be utilized effectively as a gettering site of heavy metals and increase the mechanical strength of the substrate, and thus they are considered as indispensible impurities at present, and control of these crystal defects is very important.
Accordingly, to control these crystal defects, various techniques have been tried to produce a wafer where density or size of grown-in defects is controlled, and for example, published Japanese patent application JP-A-2006-312576 discloses production of a silicon single crystal, in which a defect-free region, where grown-in defects do not exist, is enlarged over the whole wafer surface, while still having sufficient BMDs which provide gettering action in the inside, by setting the nitrogen concentration in the single crystal at from 1×1012 to less than 5×1014 atoms/cm3 and by setting hydrogen partial pressure in the gas inside the growth apparatus at less than or equal to 40 to 400 Pa, in the course of producing the silicon crystal.
According to a production method of the above published application it is disclosed that as a result of indispensably requiring production under hydrogen partial pressure, and by doping of carbon or the like so as to suppress formation of the ring-like OSF appearing in the crystal, the allowable upper limit of oxygen concentration can be increased, without elicitation of OSF nuclei. Therefore, a wafer can be produced which is composed of a defect-free region, without decreasing device characteristics, even when the oxygen concentration is high. In addition, as is also described in the published application, the relevant defect-free region includes an oxidation induced stacking fault (hereafter may be referred to simply as “OSF”) region, a PV region (a defect-free region where vacancy point defects are dominant) and a Pi region (a defect-free region where interstitial silicon point defects are dominant).
Published Japanese patent application JP-A-2005-142434 discloses a production method for a silicon single crystal wafer obtained by slicing a silicon single crystal doped with carbon and nitrogen, and then heat treating the wafer at a temperature of 900° C. up to the melting point of the silicon single crystal, under a mixed gas atmosphere.
In JP-A-2006-312576, as shown in paragraphs [0048] to [0052] and FIG. 4, it has been confirmed that by pulling-up the silicon crystal using a growing apparatus having a hot zone causing the temperature gradient of the single crystal just after solidification in the pulling direction to be smaller at the crystal peripheral part (Ge) than at the crystal center part (Gc), and growing under a predetermined hydrogen partial pressure, the Pi region widens. However, the Pi region is not adequate for applications where IG is required, because of the inability to form BMDs. Moreover, doping with nitrogen or carbon in a predetermined concentration does not lead to a significant improvement, since the Pv region hardly changes by the doping of carbon, and the Pv region does not change by the doping of nitrogen, even when the hydrogen partial pressure is over 160 Pa. Therefore, in the production method described in JP-A-2006-312576 there is a problem that widening of the Pv region cannot be controlled effectively.
In addition, in the above JP-A-2005-14234, a silicon single crystal wafer with high crystallinity and high thermal conductivity is produced by decreasing the size of grown-in defects as much as possible by doping with nitrogen and carbon, and then by performing heat treatment of the silicon wafer so as to decrease defects in particular at the surface layer portion of the wafer. However, the problem remains that sufficient IG cannot be obtained due to a low density of oxygen precipitates.
In addition, the occurrence of voids (holes formed by aggregated vacancy point defects) is less in particular in the OSF region. Such an OSF region is characterized by the absence of crystal defects which deteriorate gate oxide integrity (GOI) of an oxide film, namely C-mode characteristics (high C-mode pass rate). In such a nitrogen-doped crystal, a region having relatively low void density (specifically, a region having a void density of over 1×103/cm3 and less than or equal to 5×103/cm3 in the nitrogen-doped crystal) is present which has inferior high C-mode pass rate. Therefore, the nitrogen doped crystal disclosed in the above JP-A-2005-142434 cannot be said to have a high C-mode pass rate as a whole, and is inferior in GOI of the oxide film.